Conventional heat engines use compressible gas or vapor at high temperatures and pressures to obtain reasonable Carnot thermal efficiencies. The high temperature heat source is usually obtained from combustion of a fuel. However, there are very large sources of heat energy available at temperatures too low for efficient use in these engines. The discovery of the shape memory properties of the Ni-Ti alloy known as 55-Nitinol in 1962 opens the possibility of a solid state heat engine that can operate at quite low temperatures. The alloy can be plastically deformed to a limit of about 8% while in the low-temperature, low-strength phase, and then heated only 15.degree. or 20.degree. F to cause it to transform into another crystal structure with a strength several times greater. In addition, when the metal transforms, it "remembers" its original shape and attempts to return to it with the greater force. Upon cooling through the transformation range, the alloy regains its lower strength condition. Such an engine operating, for example, with a transformation temperature range (TTR) of 85.degree.-70.degree. F would have a Carnot thermal efficiency of only ##EQU1##